In recent years, mobile type electronic apparatuses, such as a mobile phone and a notebook personal computer, have played an important role in the information society. Those electronic apparatuses are required to be driven for a long time, and inevitably, a secondary cell functioning as a drive power source has been desired to have a higher energy density. As power sources for those electronic apparatuses and transport apparatuses, such as a vehicle, a light-weight lithium-ion secondary cell capable of having a high energy density is required to have higher performance.
A lithium-ion secondary cell has the structure in which an electrolytic solution containing a non-aqueous solvent and a lithium salt dissolved therein or a lithium solid electrolyte is provided between a negative electrode active material and a positive electrode active material, and lithium ions are moved back and forth between the negative electrode active material and the positive electrode active material and are intercalated in the active material applied on a collector substrate of a negative electrode, so that charge and discharge can be performed.
As the negative electrode active material for a lithium-ion secondary cell, although amorphous carbon having a relatively low crystallinity was used when a lithium-ion cell was first introduced on the market, in recent years, an artificial graphite material which has a high specific gravity and which is likely to obtain a high energy density has been mainly used. In general, graphite grains or carbon nanotubes, each having a high crystallinity, are mixed with a binder and then applied to a collector substrate
Although a carbon nanotube is categorized into a one-dimensional carbon nanostructure which is grown in one direction from a substrate surface by a plasma CVD method or the like, a carbon nanowall has been known which is categorized into a two-dimensional structure grown in a sheet shape on a substrate surface in a direction perpendicular thereto with a graphite layer or an amorphous layer provided therebetween (PTLs 1 to 4 and NPLs 1 and 2). PTL 4 has proposed that in a method for manufacturing carbon nanowalls in which carbon nanowalls are formed on a substrate surface in a plasma atmosphere of hydrogen and a raw material substance containing carbon and fluorine as constituent elements, by addition of oxygen atom radicals or radicals of molecules containing oxygen to the plasma atmosphere, the crystallinity of the carbon nanowall (CNW) is improved.
The carbon nanowall is a crystal which is formed from nano-size graphite crystallites and which has a relatively high completeness. The carbon nanowall is a plate-like nanostructure which is formed of approximately several to one hundred of graphene sheets overlapped with each other and which has a thickness of several to several tens of nanometers. Although a wall height of the carbon nanowall is increased to several hundreds to several thousands of nanometers in proportion to a growth time, the growth of a wall thickness is saturated up to approximately 40 nm.
In the growth of the carbon nanotubes (CNT), although the presence of catalytic metal particles, such as iron or cobalt, on a substrate surface is essential, in the case of the carbon nanowalls, catalytic metal particles are not particularly required. It has been known that by using a plasma CVD apparatus, when a carbon source gas is supplied at a substrate temperature of approximately 400° C. to 500° C. and an in-chamber pressure of approximately 100 Pa or less, the carbon nanowall grows on a substrate selectively in a direction opposite to that in which an active species effective for the grown comes down.
As a negative electrode material to be used for improvement in high-speed charge/discharge characteristics of a lithium-ion secondary cell, it is considered that the carbon nanowall has an ideal structure, and attention has been paid thereto as the negative electrode material (NPL 3 and PTLs 5 and 6). PTL 5 has disclosed that the carbon nanowall is a micrographite having a height of several micrometers and can be obtained by a vapor phase growth method without using catalysts in which a raw material gas is supplied onto a substrate at a temperature of 700° C. to 1,000° C. and a pressure of 0.5×10−3 to 1.0×10−2 Torr.
In addition, PTL 6 has disclosed a rapid charge/discharge type thin lithium cell in which flaky carbon nanowalls formed of aggregates of oriented crystallites each having a size of 10 to 30 nm are vapor-phase grown on a collector substrate to stand in a direction perpendicular thereto and are used as the negative electrode without any modification.
However, in a negative electrode using graphite, such as carbon nanowalls, the number of lithium atoms which can be intercalated between graphite layers is one atom per six carbon atoms, and the maximum charge-discharge capacity is limited to 372 mAh/g.
Amorphous hard carbon has also been known as a carbon material of the negative electrode of a lithium-ion secondary cell. However, although having a charge capacity exceeding the theoretical capacity of graphite, the hard carbon has various shortcomings, such as a large irreversible capacity and a small discharge capacity per volume.
Accordingly, silicon theoretically capable of obtaining a charge-discharge capacity exceeding that of a carbon-based negative electrode material, an alloy primarily containing silicon, a silicon oxide, and the like have drawn attention as the negative electrode material. The reason for this is that since silicon can be used as the negative electrode active material by forming an alloy with lithium and can also incorporate a large number of lithium atoms as compared to that of graphite, an increase in capacity of a lithium-ion secondary cell can be expected (for example, see NPL 4 and PTLs 7 to 9).
However, although silicon is a material having a significantly high capacity as compared to that of a carbon material, the volume of alloyed silicon which occludes lithium ions is increased by approximately 4 times that of silicon before occlusion. Hence, a negative electrode using silicon as the negative electrode active material is repeatedly expanded and contracted in synchronism with charge discharge cycles, and as a result, the negative electrode active material is mechanically destroyed. When silicon is used as the negative electrode active material of a lithium-ion secondary cell, in particular, the negative electrode active material is seriously degraded by the charge discharge cycles described above, and when charge and discharge are repeatedly performed several times, the cell capacity is almost lost.
Accordingly, as a method to overcome the shortcoming as described above, there have been developed a negative electrode for a lithium-ion secondary cell in which after a carbon nanostructure layer is formed by applying a slurry of carbon nanofibers or carbon nanotubes to electrically conductive collector foil formed of copper, titanium, nickel, or the like, followed by firing, a silicon layer having a thickness of 100 to 500 nm is further formed on the above carbon nanostructure layer by sputtering to form a composite nanostructure layer containing silicon and carbon (NPL 5 and PTL 10) and a negative electrode for a lithium-ion secondary cell in which a film of nanoscale silicon grains is deposited on surfaces of carbon nanotubes (PTL 11).
Furthermore, a negative electrode material has also been proposed in which a negative electrode active material formed of grains or coating films of silicon or the like is fixed to vertical wall surfaces of graphene sheets of carbon nanowalls formed on a collector substrate, and the change in volume of the negative electrode active material in synchronism with charge discharge cycles is reduced by the spaces between the graphene sheets of the carbon nanowalls so as to increase the capacity (PTL 12 and NPL 6).